Technique for stabilizing laser wavelength and phase

ABSTRACT

A technique to stabilize the effective refraction index of a laser generating system&#39;s wave guide, as well as a technique to stabilize the phase of the wave guide. In at least one embodiment of the invention, a polymer is used within the wave guide to counteract the effects of temperature on the clad material of the wave guide in order to create an overall effective refraction index that is substantially independent of temperature variations. Furthermore, in at least one embodiment of the invention relative segment lengths of the wave guide are chosen to stabilize the phase of the wave guide.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 10/425,279, filed Apr. 28, 2003, and claimspriority thereto under 35 U.S.C. § 120.

FIELD

[0002] Embodiments of the invention relate to laser technology. Moreparticularly, embodiments of the invention relate to stabilizing alight's wavelength or phase across multiple wave guide temperatures byusing materials within the wave guide having varying temperaturereaction characteristics.

BACKGROUND

[0003] Laser systems use focused, intense light rays of a particularwavelength or wavelength range that may be used in various applications,including data storage, medicine, semiconductor processing, and networkcommunications. The light used in laser systems, however, can besensitive to temperature variations among structures within the lasergenerating system. This is due, at least in part, to temperaturesensitivity of light refraction indices in various materials used withinthe laser generating system.

[0004]FIG. 1 illustrates a prior art system for generating a light in alaser system. The light is generated and amplified by a semiconductoroptical amplifier (SOA) chip 101. The light generated by the SOA entersa wave guide 105 consisting of a clad material 110, such as silicondioxide or other material with a lower refractive index than the core, awave guide core 115, and series of grating elements 120 (grating) thathelp to direct and refine the light toward a desired wavelength andphase. After light enters the wave guide, it is passed through thegrating, which can refine the character of the light, including thelight's wavelength.

[0005] One way in which the grating can refine the wavelength of thelight is to reflect certain wavelengths in the light and propogateothers. Particularly, the grating can refine the light's wavelength byreflecting the undesired wavelengths of the light from the gratingtoward the SOA chip where the light can be amplified and re-directtoward the grating. Moreover, a desired light phase can be produced byplacing the grating a certain distance from the SOA chip, such that theround trip distance of the reflected light is an integer division of thedesired wavelength.

[0006] Unfortunately, the wave guide clad material and the wave guidecore material can change temperature during the course of generating andrefining the light, which can, in turn, change the refraction indices ofthe wave guide core and clad materials. The refraction index of amaterial is an indicator of the material's ability to pass or reflectcertain frequencies of light. As the refraction index of the clad orcore material changes with temperature, less of a particular wavelengthof light may be reflected and therefore propagated through the waveguide, resulting in loss of light intensity or a change in the light'swavelength.

[0007] As a light travels through the wave guide core, it can beeffected by the overall effective refraction index of a substantiallycylindrical area surrounding the wave guide core known as the opticalmode. FIG. 2 illustrates a cross-sectional view of the wave guide, inwhich the cross-section of the optical mode is circumscribed by thecircle 201. The material within the boundary of the optical mode caneffect the light traveling through the core if the temperature of thematerial changes, due to the resulting change in the refraction index ofthe material within the optical mode.

[0008] Adverse effects on the light due to temperature sensitivity ofrefraction indices of materials has been addressed in some prior artlaser generating systems by using power-consuming devices, such as athermal electric cooler (TEC). The TEC may be used to cool the waveguide within the optical mode as the wave guide temperature increasesfrom the laser generation process. Through, what can be, an elaboratetechnique of detecting the optical mode temperature and adjusting theTEC accordingly, the temperature of the wave guide in the optical modecan remain stable enough to generate a light that is substantially thedesired wavelength and phase for a particular application.

[0009] The TEC, however, can have adverse effects on system powerconsumption, system cost, and system reliability. Furthermore, theaccuracy of the light's wavelength and phase, using a TEC, is, at leastin part, a function of how quickly the TEC can respond to temperaturevariations within the optical mode without over-compensating for thosevariations. As a result, the overall accuracy of the light can becompromised.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Embodiments of the invention are illustrated by way of exampleand not limitation in the figures of the accompanying drawings, in whichlike references indicate similar elements and in which:

[0011]FIG. 1 illustrates a prior art laser generating system.

[0012]FIG. 2 illustrates an optical mode within a prior art lasergenerating system.

[0013]FIG. 3 illustrates a laser generating system according to oneembodiment of the invention, in which a polymer is added to the waveguide.

[0014]FIG. 4 illustrates a cross-sectional view of a laser generatingsystem according to one embodiment of the invention in which a polymerexists within the optical mode on opposite sides of the wave guide.

[0015]FIG. 5 illustrates a cross-sectional view of a laser generatingsystem according to one embodiment of the invention in which a polymerexists within the optical mode on one side of the wave guide.

[0016]FIG. 6 illustrates a laser generating system according to oneembodiment in which a phase is substantially maintained acrosstemperature variances.

DETAILED DESCRIPTION

[0017] Embodiments of the invention pertain to the generation of a lightof a desired wavelength and phase. More particularly, embodiments of theinvention pertain to using certain materials within the wave guide of alaser generation system, such that the refraction indices of thematerials contribute to an overall substantially constant effectiverefraction index of the optical mode of the wave guide, which is largelyindependent of temperature variations.

[0018] Stabilization of a light wavelength within a laser generatingsystem can be achieved more reliably, accurately, and inexpensively thanin many prior art techniques by introducing a material into the waveguide clad that has inverse refraction characteristics to those of theclad material across temperature variances.

[0019] For example, the refraction index of the clad material mayincrease with temperature, thereby causing the overall effectiverefraction index of the wave guide within the optical mode to increase,which may then effect the wavelength, intensity, or othercharacteristics of the light produced. Adding a material to the waveguide within the optical mode whose refraction index decreases withincreasing temperature can help to counter this effect, creating anoverall effective refraction index within the optical mode that issubstantially the same across varying temperatures.

[0020]FIG. 3 illustrates one embodiment of the invention in which apolymer is added to the wave guide clad material in order to offset theeffect of temperature-induced refraction index changes of the cladmaterial within the optical mode. In FIG. 3, a polymer 301 existsanywhere throughout the grating area 305 in order to stabilize theeffective refractive index of the clad 310 across the grating region.This is but one example, however, of where the polymer may be placedwithin the clad in order to have the desired effect. Furthermore, thepolymer may be placed throughout the clad in various positions andquantities depending upon the clad material used and the particulardesign needs of the laser generating system.

[0021] A light's wavelength is stabilized across varying cladtemperatures in at least one embodiment of the invention, by usingappropriate proportions of clad material and polymer within the opticalmode of the wave guide.

[0022]FIG. 4 illustrates a cross section of a wave guide according toone embodiment of the invention, in which the polymer and clad materialexist within the optical mode of the wave guide in suitable proportionsto have the desired stabilizing effect. The cross-sectional area of thepolymer 401 that exists within the optical mode 403 in order toeffectively offset the temperature-induced variations of the refractionindex of the clad material 405 is determined in the embodimentillustrated in FIG. 4 by the equation:$n_{{eff},{grating}} = \frac{{n_{polymer} \cdot a_{polymer}} + {n_{core} \cdot a_{core}} + {n_{clad} \cdot a_{clad}}}{a_{polymer} + a_{core} + a_{clad}}$

[0023] In the above equation, the effective refraction index within thegrating region is a function of the multiplicative product of therefractive indices (n_(polymer), n_(core), and n_(clad)) of the variousmaterials within the optical mode and the areas (a_(polymer), a_(core),and a_(clad)) of the optical mode that they occupy.

[0024] In other embodiments of the invention, other methods ofdetermining the proportion of clad, core, and polymer and their relativepositions in the optical mode in order to stabilize the effectiverefraction index of the laser generating device may be used. Forexample, FIG. 5 illustrates one embodiment of the invention in which thepolymer exists on only one side of the core. However, the proportion ofareas of core 510, polymer 501, and clad 505 within the optical mode 515are such that the above equation is satisfied.

[0025] The desired wavelength that is passed by the grating in theembodiment illustrated in FIG. 4 is determined by the equation:

λ₀ n _(eff,grating)·Λ

[0026] In the above equation, the effective grating refraction index atthe grating region is multiplied by the period of the grating, denotedby the upper-case lambda. The period of the grating is determined inpart by the periodic modulation of the portion of the optical mode'seffective refraction index that surrounds the grating length. Within thelength of the grating, one half of the period of the light that passesthrough it has a slightly higher index than the other half due to thechanging effective refraction index from one end of the grating to theother. Because of this small index difference, each lens of the gratingbehaves like a weak mirror, partially reflecting the light as it passesthrough. Therefore, the period of the grating is a function of thethickness of each lens.

[0027] In addition to the wavelength of the light, the phase of thelight may be adversely effected by temperature changes within theoptical mode of the wave guide. This effect can occur, for example, ifthe refraction index changes within the optical mode of the portion ofthe wave guide in which the light is reflected by the grating. Forexample, if the round-trip optical length of a photon (quantum of thelight's energy embodied in a range of wavelengths) of the light thatreflects back from the grating to the light source does not have awavelength that is an integer division of the desired light wavelength,then destructive effects can occur to the desired light photon.

[0028]FIG. 6 illustrates a laser generating system according to oneembodiment of the invention, in which a polymer segment has beenintroduced to the clad material of the wave guide between the SOA chipand the grating. The polymer segment 601 extends into the optical modeof the wave guide enough to satisfy the above equations relating towavelength stability and is of an appropriate length along the waveguide core to create a stable light phase that is substantiallyindependent of temperature variations.

[0029] A desired optical round trip distance traveled by the photonsreflected back to the SOA chip from the grating is maintained in theembodiment illustrated in FIG. 6 by choosing relative lengths of theclad material segments and the polymer segments that satisfy thefollowing equation:

2(n _(eff,SOA) ·L _(SOA) +n _(eff,L1) ·L1+n _(eff,phase) ·L _(phase) +n_(eff,L2) ·L2+n _(eff,grating) ·L _(grating)/2)=m·λ ₀

[0030] In the above equation, the summation of the multiplicativeproducts of the refraction indices of the various segments between theSOA chip and the grating (n_(eff,SOA), n_(eff,L1), n_(eff,phase),n_(eff,L2), and n_(eff,grating)) and the lengths of the respectivesegments (L_(SOA), L_(L1), L_(phase), L_(L2), and L_(grating)) 602, 603,604, 605, 606 are a constant integer multiple (m) of the desired lightwavelength 607. The entire sum is multiplied by two to account for theround trip of the light photon. In other embodiments, other methods ofdetermining the length of clad, core, and polymer segments in order tostabilize the effective phase of the laser generating device may beused. For example, in at least one embodiment of the invention, L_(L1),L_(phase), and L_(L2), may be represented by one or two lengthsencompassing the the sum of L_(L1), L_(phase), and L_(L2). Furthermore,one or more of the these segment lengths may be represented by multiplesegment lengths in other embodiments of the invention.

[0031] Also illustrated in FIG. 6 is a graph 608 showing the emissionspectrum power of the SOA chip and a graph 609 showing the reflectivitypercentage of the grating at various wavelengths along the lightspectrum, according to one embodiment of the invention. The graph 609indicates that the grating effectively passes the highest power spectralrange of the light and reflects the rest in at least one embodiment ofthe invention.

[0032] Although a polymer is used in the above embodiments of theinvention, other materials may be used in addition to or instead of thepolymer that possess refraction indices suitable to stabilize theeffective refraction index and/or phase of a particular clad material.Furthermore, the distribution, concentration, and position of thepolymer or other material(s) are different in other embodimentsdepending in part upon the physical characteristics of the clad and thelaser generating system. Similarly, the SOA chip is only one example ofa light source that may be used with embodiments of the invention. Otherlight sources, including those integrated within the wave guide, may beused in other embodiments of the invention.

[0033] While the invention has been described with reference toillustrative embodiments, this description is not intended to beconstrued in a limiting sense. Various modifications of the illustrativeembodiments, as well as other embodiments, which are apparent to personsskilled in the art to which the invention pertains are deemed to liewithin the spirit and scope of the invention.

What is claimed is:
 1. An apparatus comprising: a wave guide cladcomprising a first material whose refraction index varies by a firstmagnitude according to a temperature variation of the first material anda second material whose refraction index varies by a second magnitudeaccording to the temperature variation of the second material, thesecond magnitude being inversely related to the first magnitude; a waveguide core within the wave guide clad.
 2. The apparatus of claim 1wherein the first magnitude contributes to an increase in the refractionindex of the first material in response to the temperature variation andthe second magnitude contributes to a decrease in the refraction indexof the second material in response to the temperature variation.
 3. Theapparatus of claim 2 further comprising a source for producing light. 4.The apparatus of claim 3 further comprising grating to reflect a portionof the light as the light passes through the wave guide core.
 5. Theapparatus of claim 4 wherein the second material is a polymer thatexists within the grating area.
 6. The apparatus of claim 1 whereinportions of the first material and the second material contribute to aneffective refraction index of the wave guide clad.
 7. The apparatus ofclaim 6 wherein the refraction index of the wave guide clad depends uponthe relative amount of each of the first and second materials within theoptical mode of the wave guide clad.
 8. An apparatus comprising: firstmeans for stabilizing a light's wavelength within a wave guide, thefirst means comprising two materials, each having a refraction index tochange in opposite magnitude in relation to the other in response tovariations in temperature of the wave guide.
 9. The apparatus of claim 8wherein variations of the light's wavelength in response to temperaturevariations of the wave guide depends upon the amount of one of the twomaterials in relation to the other within an optical mode of the waveguide.
 10. The apparatus of claim 9 wherein one of the two materials isa polymer.
 11. The apparatus of claim 10 wherein the polymer exists onopposite ends of a grating within the wave guide.
 12. The apparatus ofclaim 8 further comprising a second means for stabilizing the phase ofthe light across varying temperatures of the wave guide.
 13. Theapparatus of claim 12 wherein the second means comprises the twomaterials in proportionate amounts so as to make a round-trip refractiondistance of a photon of the light substantially independently oftemperature.
 14. The apparatus of claim 13 wherein one of the twomaterials is a polymer and one of the two materials is clad material.15. The apparatus of claim 14 wherein the effective refraction index forthe wave guide is dependent upon the product of a length of a polymersegment and the refraction index of the polymer.
 16. An apparatuscomprising: first means for stabilizing a light's phase within a waveguide, the first means comprising two materials, each having arefraction index to change in opposite magnitude in relation to theother in response to variations in temperature of the wave guide. 17.The apparatus of claim 16 wherein one of the two materials is a polymerdistributed in segments along the length of a wave guide core within thewave guide.
 18. The apparatus of claim 17 wherein the light's wavelengthdepends upon the length of the segments multiplied by an effectiverefractive index of each segment, the effective refractive index of eachsegment depending upon an amount of the polymer distributed within anoptical mode of the wave guide.
 19. The apparatus of claim 18 wherein aneffective refraction index of the wave guide is substantially constantfrom a first end of the wave guide through a grating of the wave guide.20. The apparatus of claim 19 wherein the light is produced by a sourceexternal to the wave guide.
 21. The apparatus of claim 19 wherein thelight is produced by a source internal to the wave guide.
 22. Theapparatus of claim 20 wherein the source of the light is a semiconductoroptical amplifier (SOA) chip.
 23. The apparatus of claim 22 wherein thewavelength of the light substantially corresponds to the maximum powerwithin the emission spectrum of the SOA.
 24. A system comprising: alight source to emit a spectrum of light wavelengths; a wave guide toguide light from the light source having a first wavelength, the waveguide comprising a clad material, the wave guide including a polymer tohelp maintain an effective wave guide refraction index within an opticalmode of the wave guide that is independent of temperature changes in thewave guide.
 25. The system of claim 24 wherein the refraction index ofthe polymer changes in opposite magnitude of a clad material of the waveguide in response to temperature variations.
 26. The system of claim 25wherein the effective refraction index of the polymer depends upon therelative amounts of polymer and other clad material existing within theoptical mode of the wave guide.
 27. The system of claim 26 wherein thephase of the light is substantially independent of temperaturevariations within the wave guide.
 28. The system of claim 26 wherein thewavelength of the light is substantially independent of temperaturevariations within the wave guide.
 29. The system of claim 26 wherein thewave guide comprises a grating to reflect light wavelengths within thespectrum emitted by the light source.
 30. The system of claim 29 whereinthe light source comprises a semiconductor optical amplifier (SOA) toamplify the reflected light wavelengths.